(1) Field of the Invention
This invention relates to an optical laminate film, which can be easily produced and suitable for compensating birefringence, and a method for producing the optical laminate film.
(2) Description of the Related Art
A liquid crystal display device utilizing birefringence of super twisted nematic (STN) type for high-contrast image is used for a variety of image displays such as in personal computers, word processors or the like. However it is known that the birefringence of liquid crystal cells causes problems in the view angle characteristic of the liquid crystal display device using a twisted nematic liquid crystal, cholesteric liquid crystal or smectic liquid crystal, that is, when the image on the screen is viewed straight on, the image looks good, but when the image is viewed from an oblique view, undesirable discoloration and disappearance of image are liable to occur. To solve this problem, a retardation plate disposed between a liquid crystal cell and polarizing plate has been widely adopted as the conventional approach.
Japanese Unexamined Patent Application Publication (hereinafter referred to as “JP-A”) H2-160204 discloses a retardation film characterized by satisfying the formula: 0.92≦(R40/Re)≦1.08, wherein Re represents a retardation in the case when a monochromatic light with wavelength 632.8 nm is incident perpendicularly onto a film, and R40 represents a retardation in the case when a monochromatic light with wavelength 632.8 nm is incident on the film at an angle of 40 degree to the normal line of the film surface.
U.S. Pat. No. 5,245,456 discloses a birefringent film characterized by a mixture of molecules oriented in the plane direction of the film and molecules oriented in a thickness direction. This US patent further discloses a process for producing the birefringent film characterized in that a shrinkable film is adhered onto one or both surfaces of a resin film to form a laminate film, and then the laminate film is stretched while being heated to impart a shrinking force exerting in the direction perpendicular to the direction in which the resin film is stretched.
U.S. Pat. No. 5,189,538 discloses a liquid crystal display device which includes a liquid crystal cell, a polarizing plate, and at least one film (A) and at least one film (B), which are disposed between the liquid crystal cell and the polarizing plate, wherein the film (A) has light transmittance and meets the condition that the film satisfies the inequality [NTH−(NMD−NTD)/2]>0, where NTH represents a refractive index in the direction normal to the film surface, DMD represents a refractive index in the longitudinal direction (i.e., machine direction) of the film, and NTD represents a refractive index in the transverse direction of the film; and the film (B) is a uniaxial-stretched film made of a polymer having a light transmittance and a positive intrinsic birefringence.
However, there are problems when the film has been made by the above disclosed methods. For example, in the case of the method disclosed in JP-A H2-160204, the dispersion of retardation is large and the production efficiency is low. Also this method has difficulty in producing a large sized product, which can be applied to a large sized liquid crystal screen such as a high definition TV. In the case of the process disclosed in U.S. Pat. No. 5,245,456, precise control of the ratio of stretching to shrinkage is required, which leads to a complicated production process and results in reduction of productivity. In the case of the liquid crystal display device disclosed in U.S. Pat. No. 5,189,538, it appears that the film used for the liquid crystal display device is produced easily and the retardation can easily be controlled, particularly when the biaxially-stretched film or the uniaxially-stretched film made of material of negative intrinsic birefringence value is used as the film (A). In practice, it is difficult to make a retardation film by stretching the film having a negative intrinsic birefringence value, such as a vinyl aromatic polymer film, which is preferable in terms of having a large absolute value of the intrinsic birefringence and good transparency. In order to manifest a preferable retardation and hold a uniform retardation of the film, it is necessary to carry out a uniaxial longitudinal stretching or uniaxial transverse stretching with zone heating, or sequential or simultaneous biaxial stretching. However, film-breaking occurs easily during the stretching step because of insufficient strength of the material. Stretching under a high temperature to avoid breaking makes it difficult to obtain the preferred retardation, and gives easily non-uniform retardation. There seems to be no retardation film from a practical standpoint that is made of a material having a negative intrinsic birefringence value and satisfying the inequality [NTH−(NMD−NTD)/2]>0.
A retardation film, in which there is substantially no retardation and yet a refractive index in the thickness direction is larger than that of the plane direction, i.e., a positive-retarder, is obtained by carrying out biaxial stretching of the film made of the material with negative intrinsic birefringence value, which can be, for example, expected to be applied to phase-difference compensation film used for display devices using cholesteric liquid crystal. However, a film-breaking still occurs easily at the stretching step because of insufficient strength of the material and because stretching under a high temperature to avoid the breaking makes it difficult to obtain the preferred retardation and leads easily to non-uniform retardation, and therefore, there seems to be no such retardation film from a practical standpoint.